1. Field of the Invention
The present invention relates generally to a method and device for detecting and/or treating biological contaminants in an electrodeposition coating process to provide electrocoated substrates having good smoothness and appearance.
2. Technical Considerations
Electrodeposition has become the primary method for applying corrosion-resistant primers in automotive applications. The electrodeposition process typically involves cleaning and/or pretreating an electroconductive substrate and then immersing the substrate into an electrocoating tank containing a bath of an aqueous electrocoating composition. The substrate serves as a charged electrode in an electrical circuit comprising the electrode and an oppositely charged counter-electrode. Sufficient current is applied between the electrodes to deposit a substantially continuous, adherent film of the electrocoating composition onto the surface of the substrate.
The electrocoated substrate is then conveyed into a rinsing operation where it is rinsed with an aqueous rinsing composition. Typical rinsing operations have multiple stages which can include closed loop spray and/or dip applications. In a spray rinse application, the electrocoated substrate exits the electrocoating tank and is conveyed over a rinse tank while an aqueous rinsing composition is spray applied by a spraying apparatus to the electrocoated surfaces of the substrate. Exces rinsing composition is permitted to drain from the substrate into the rinse tank below. The rinsing composition in the rinse tank is then recirculated back to the spraying apparatus for subsequent spray applications. In a dip rinse application, the electrocoated substrate is conveyed into a dip tank, where it is immersed in an aqueous rinsing composition, and is subsequently conveyed through one or more spray rinse applications as described above.
Recirculating the coating or rinsing compositions is both economically and environmentally desirable. However, the combination of organic nutrients, warmth, and recirculation in an aqueous coating system creates an environment conducive to bacterial and/or fungal growth. These biological contaminants, if left unchecked, could adversely affect the quality and appearance of the electrodeposited coating. Biological contaminants present in the coating and/or rinsing compositions can cause pH shifts, particulate “dirt” deposition, and biofouling, which may detrimentally affect the appearance of the coating and reduce system performance.
In order to combat such biofouling, biocides or biological inhibitors, such as ethylene glycol monobutyl ether, and the like, are typically added to liquids in the coating process, such as coating and/or rinsing compositions, when undesirable levels of microbial activity are detected. Since such materials can be environmentally undesirable as well as expensive, they are typically used sparingly and only when the measured biological activity begins to reach a point where it will adversely impact upon the coating process.
The presence and/or level of biological contaminants in a coating system, e.g., a coating composition, is typically measured by taking a sample of the coating composition and shipping the sample to a laboratory for analysis. However, this current practice has some undesirable limitations. For example, the analysis requires a certain degree of technical skill and is typically carried out at an off-site facility, requiring time for the sample to be taken, shipped, analyzed, and the results of the analysis returned. Therefore, the results obtained are not “real time” and the actual level of biological contamination present in the coating composition, and hence the correct amount of biocide to add, must be estimated based on the time at which the analyzed sample was actually taken. Further, such analysis is relatively costly to perform.
In non-coating related fields, non-analytical methods of measuring biological contaminants have been developed. For example, U.K. Patent Application GB 2,319,837 discloses a method of determining biological activity in a soil sample by placing the soil sample in a sample chamber linked to an indicator chamber, the sample chamber and indicator chamber being isolated from the ambient atmosphere. U.S. Pat. Nos. 5,094,955; 5,518,895; 5,856,175; and 5,858,769 disclose a device for detecting the presence of microorganisms in clinical specimens, such as blood or other body fluids, by culturing the specimens with a sterile growth medium in a transparent, sealed container. The presence of microorganisms in the sealed container is determined by measuring changes in the pH of the specimen or the production of CO2 within the sealed container using a sensor affixed to the interior surface of the sealed container. However, such methods cannot be directly translated into the coating field, particularly the continuous electrodeposition coating field, since the coating tanks and rinse tanks in a conventional coating operation realistically cannot be isolated from the ambient atmosphere during the coating operation.
It would be desirable to provide a method and device for detecting and/or controlling the level of biological contaminants in coating and/or rinsing compositions of a coating process, and more particularly in electrodeposition coating and/or rinsing compositions, without adversely impacting upon operation of the coating and/or rinsing operations.